This invention relates to a method of preparing refined starch hydrolysates from starch-containing cereals.
The term "starch-containing cereals" is herein to be understood as comprising wheat, barley and rye.
The starting material ordinarily used in the production of starch hydrolysates is refined maize starch. Refined maize starch is produced by steeping maize kernels in steeping water containing about 0.2% SO.sub.2 and subsequently wet-milling the steeped product so as to effect a release of the starch granules. The starch is subsequently separated from proteins, germs and fibres on high speed centrifuges, hydrocyclones, screens or filters.
The refined maize starch thus produced is then hydrolyzed so as to form a hydrolysate mainly consisting of dextrose. The hydrolysis may be catalyzed by an acid or by enzymes or by a combination of acid and enzymes.
Finally, the hydrolysate is refined by filtration and/or by treatment with an ion exchanger material or activated carbon.
The use of cereals as starch-containing starting material presents some problems which are not met in the ordinary maize starch hydrolysis.
Like maize, wheat, barley and rye contain other carbohydrates than starch, such as pentosans, which are present in wheat, barley and rye in an amount of from 1.5 to 3% by weight. However, whereas the pentosans present in maize are insoluble and remain insoluble during the hydrolysis and consequently can be easily removed from the starch hydrolysate, the pentosans contained in wheat, barley and rye tend to be solubilized when heated in the presence of water and create serious problems during the further processing of the hydrolysate.
Thus, due to the relatively low molecular size of the solubilized pentosans it is difficult to separate these pentosans from the starch hydrolysate produced. On the other hand they cannot be allowed to remain in the starch hydrolysate to be refined, because of their adverse effect on the filtration rate of the ultrafiltration process and/or the energy consumption of such a process. Furthermore, the presence of solubilized pentosans in an ultrafiltration permeate makes the concentration of the permeate difficult. Thus, if the permeate contains pentosans in a concentration of 10-12% by weight, the permeate tends to be converted into a gel when being concentrated.
In order to eliminate the problems caused by the pentosans, it has been proposed (F. J. Simpson, Canadian Journal of Technology, 33, 33-40, 1955) to hydrolyze pentosans present in starch slurries prepared from wheat flour by using pentosanases from specific Bacillus strains.
This solution has not been successful, because it is based on use of flour, i.e. a finely milled product, and because the milling operation results in the formation of so-called "B- Starch", i.e. starch which is very difficult or even impossible to convert into a hydrolysate.
The present invention is based on the surprising discovery that the pentosans contained in wheat, barley and rye are closely attached to the fine fibres of such cereals, and that these fibres and the pentosans attached thereto can be separated from the starch-containing product, provided the starting material is hydrated and softened and subsequently wet-milled in a gentle manner.
Furthermore, it has been found that if the starch-containing product obtained after separation of fibres and pentosans attached thereto as well as germs and gluten from the hydrated and softened and subsequently wet-milled starting material is carefully washed with water, soluble pentosans, if any, can be removed therefrom so as to obtain a refined starch product essentially free of pentosans.